Bichromal balls have two hemispheres, typically one black and one white, each having different electrical properties. Such bichromal balls are frequently used in a “twisting ball” medium for displaying an image. The twisting ball medium includes internal bichromal balls that rotate to show either black or white hemispheres in response to an externally applied electrical field which are contained in individual liquid filled spherical cavities in a transparent binder, such as a gel. The gel is then bonded between glass or plastic sheets for protection.
Alternatively, such bichromal balls may be enclosed within individual spherical shells and then a space between the ball and shell is filled with a liquid to form a microsphere so that the ball is free to rotate in response to an electrical field. The microspheres can then be mixed into a substrate which can be formed into sheets or can be applied to any kind of surface. The result is a film which can form an image from an applied electrical field. The display formed using this technique allows the formation of a thin, paper-like sheet without the bulkiness and optical problems created by the need for protective cover sheets in a twisting ball medium. In digital document media, the bichromal balls are embedded in a gel matrix. By applying an external electric field, the bichromal balls are rotated to direct either of their two differently colored sides to an observer.
However, with the current processes for encapsulation, a number of empty shells (i.e., shells which do not encapsulate a bichromal ball) are generated with the individual encapsulated bichromal balls. Because both the empty and filled shells have about the same mass, they can not easily be separated by typical sedimentation processes. However, since only filled shells produce an image, the inclusion of empty shells in a medium reduces the image quality. Therefore, for applications requiring high image quality, empty shells will need to be separated from a mixture containing empty shells and filled shells.